#include <iostream>
using namespace std;

// Definition for singly-linked list.
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(!head || !head->next) return nullptr;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast && fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
            if(fast == slow) break;
        }
        if(fast != slow) return nullptr;
        fast = head;
        while(fast != slow)
        {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};

int main()
{
    return 0;
}